1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst.
2. Description of the Related Art
As an exhaust gas-purifying catalyst that treats an exhaust gas of an automobile, a three-way catalyst with a precious metal such as platinum supported by an inorganic oxide such as ceria or alumina has been widely used. In the three-way catalyst, the precious metal plays the role in promoting the reduction of nitrogen oxides and the oxidations of carbon monoxide and hydrocarbons. The inorganic oxide plays the roles in increasing the specific surface area of the precious metal and suppressing the sintering of the precious metal by dissipating heat generated by the reactions.
In recent years, occasions when an automotive vehicle such as an automobile is driven at high-speed increase as the performance of an engine increases. Additionally, in order to prevent the pollution of the air, the regulations on the exhaust gas are made more stringent. Against these backdrops, temperature of the exhaust gas emitted by the automotive vehicle is on the trend of rising. For that, in order to achieve the exhaust gas-purifying catalyst that exhibits sufficient performance even when used under such a condition, research and development are actively carried out.
For example, JP-A 1-242149 describes that alumina supporting a precious metal reacts with nickel at high temperatures to produce NiAl2O4 having a spinal structure when nickel is used in order to remove hydrogen sulfide from an exhaust gas. The Patent Literature describes that the reaction greatly deteriorates the activity of a catalyst. Furthermore, the Patent Literature describes that use of a composite oxide containing ceria and zirconia is effective for the suppression of the reaction.
JP-B 6-75675 describes that ceria causes grain growth when used at high temperatures, which leads to deterioration in oxygen storage capacity. The Patent Literature describes that a composite oxide represented by a general formula Ce1-xLaxO2-x/2 having a fluorite structure also causes grain growth when used at high temperatures, which leads to deterioration in purification performance. The Patent Literature further describes that a composite oxide or a solid solution of zirconia and ceria having an atomic ratio of zirconium to cerium within a range of 5/95 to 70/30 is less prone to cause grain growth even when used at high temperatures.
JP-A 10-202101 describes that a support in which alumina, ceria, and zirconia are uniformly distributed exhibits high heat resistance.
JP-A 2004-41866 describes a composite oxide having a perovskite structure represented by a general formula. ABPdO3. In the general formula, the element A is at least one rare-earth element such as La, Nd, and Y having a valence of 3 and cannot have other valencies. The element. B is at least one element selected, from the group consisting of Al and transition elements other than Co, Pd, and rare-earth elements. The Patent Literature describes that the catalyst activity of palladium is maintained at a high level for a long period of time when the composite oxide is used.
JP-A 2004-41867 describes a composite oxide having a perovskite structure represented by a general formula ABRhO3. In the general formula, the element A is at least one rare-earth element such as La, Nd, and Y having a valence of 3 and cannot have other valencies, or a combination of such a rare-earth element and at least one of Ce and Pr. The element B is at least one element selected from the group consisting of Al and transition elements other than Co, Rh, and rare-earth elements. The Patent Literature describes that the catalyst activity of rhodium is maintained at a high level for a long period of time when the composite oxide is used.
JP-A 2004-41868 describes a composite oxide having a perovskite structure represented by a general formula A1-xA′xB1-y-zB′yPtzO3. In the general formula, the element A is at least one rare-earth element such as La, Nd, and Y having a valence of 3 and cannot have other valencies. The element A′ is at least one element selected from alkaline-earth metals and Ag. The element B is at least one element selected from Fe, Mn, and Al. The element. B′ is at least one element selected from transition elements other than Pt, Fe, Mn, Co, and rare-earth elements. The Patent Literature describes that the catalyst activity of platinum is maintained at a high level for a long period of time when the composite oxide is used.